The present invention relates to an automatic transmission, and more particularly, to an electro-mechanical automatic transmission having dual input shafts.
There are presently two typical power transmissions in use on the conventional automobile. The first, and oldest, type of powertrain is the manually operated powertrain. These powertrains are typically characterized in that vehicles having manual transmissions include a clutch pedal to the left of a brake pedal and a gear shift lever which is usually mounted at the center of the vehicle just behind the dashboard. To operate the manual transmission, the driver must coordinate depression of the clutch and accelerator pedals with the position of the shift lever in order to select the desired gear. Proper operation of a manual transmission is well known to those skilled in the art, and will not be described further herein.
In a vehicle having an automatic transmission, no clutch pedal is necessary. The standard H configuration of the shift lever is replaced by a shift lever which typically moves back and forth. The driver need only select between park, reverse, neutral, drive, and one or two low gears. As is commonly known in the art, the shift lever is placed in one of several positions having the designator P, R, N, D, 2, and maybe 1 which corresponds to Park, Reverse, Neutral, Drive, and one or two low gears, respectively. Vehicle operation when the gear shift lever is placed in one of these positions is well known in the art. In particular, when in the drive mode, the transmission automatically selects between the available forward gears. As is well known, older systems typically included first, second and third gears, while newer systems include first through third gears as well as fourth and possibly a fifth and sixth overdrive gear. The overdrive gears provide an improved fuel economy at higher speeds.
As is well known, early transmissions were almost exclusively manually operated transmissions. With a steady development of automatic transmissions, drivers increasingly gravitated toward the easy operation of automatic transmissions. However, in the mid 1970s, rising concerns about present and future fossil fuel shortages resulted in an implementation of corporation average fuel economy regulations propagated in several countries. These fuel economy requirements necessitated the investigation of increasing the fuel economy of motor vehicles in order to meet government regulations. These government regulations prompted a gradual return to manual transmissions which are typically more efficient than automatic transmissions.
In the ensuing years, many mechanically operated vehicle systems were replaced or at least controlled by electronic control systems. These electronic control systems greatly increased the fuel efficiency of vehicle engines and enabled a gradual return to the convenience of automatic transmissions. In addition, electronic controls used with automatic transmissions, greatly improved the shift schedule and shift feel of automatic transmissions and also enabled implementation of fourth and fifth overdrive gears thereby increasing fuel economy. Thus, automatic transmissions have once again become increasingly popular.
Automatic and manual transmissions offer various competing advantages and disadvantages. As mentioned previously, a primary advantage of a manual transmission is improved fuel economy. Conversely, automatic transmissions first and foremost offer easy operation, so that the driver need not burden both hands, one for the steering wheel and one for the gear shifter, and both feet, one for the clutch and one for the accelerator and brake pedal, while driving. When operating a manual transmission, the driver has both one hand and one foot free. In addition, an automatic transmission provides extreme convenience in stop and go situations, as the driver need not worry about continuously shifting gears to adjust to the ever-changing speed of traffic.
The primary reason for the superior efficiency of the manual transmission over the automatic transmission lies in the basic operation of the automatic transmission. In most automatic transmissions, the output of the engine connects to the input of the transmission through a torque converter. Most torque converters have an input turbine that is connected to the output shaft of the engine and an input impeller that is connected to the input shaft of the transmission. Movement of the turbine at the input side results in a hydraulic fluid flow which causes a corresponding movement of the hydraulic impeller connected to the input shaft of the transmission. While torque converters provide a smooth coupling between the engine and the transmission, the slippage of the torque converter results in a parasitic loss, thereby decreasing the efficiency of the powertrain. Further, the shift operation in an automatic transmission requires a hydraulic pump which pressurizes a fluid for clutch engagement. The power required to pressurize the fluid introduces additional parasitic losses of efficiency in the powertrain. In addition, other parasitic losses associated with automatic transmissions exist.
Before a shift between the gear ratios of a manual transmission can occur, it is necessary to synchronize the rotational speed of the driveshaft with the rotational speed of the driven shaft. Typically, synchronization is obtained in a manual transmission by way of a synchronizing mechanism such as a mechanical synchronizer which is well known in the art. The mechanical synchronizer varies the speed of the driveshaft to match the speed of the driven shaft to enable smooth engagement of the selected gear set. For example, during an upshift, the mechanical synchronizer utilizes frictional forces to decrease the rate of rotation of the driveshaft so that the desired gear of the driveshaft is engaged smoothly to drive the desired gear of the driven shaft. Conversely, during a downshift, the mechanical synchronizer increases the rate of rotation of the driveshaft so that the desired gear is engaged smoothly to drive the desired gear on the driven shaft. Typically, with a manual transmission, there is a delay period between disengagement of the currently engaged gear and the subsequent synchronization and engagement of the desired transmission gear. Also, during this process, the clutch connection between the engine output shaft and the transmission input shaft needs to be disengaged prior to the gear shifting process and re-engaged upon synchronization.
Thus, it is an object of the present invention to provide an electro-mechanical automatic transmission which utilizes the manual-type transmission design in order to eliminate the parasitic losses associated with the torque converter and the hydraulic controls of conventional automatic transmissions. The electro-mechanical automatic transmission of the present invention is essentially an automated manual transmission. The design utilizes a dual clutch/dual input shaft layout. The layout is the equivalent of having two transmissions in one housing. Each transmission can be shifted and clutched independently. Uninterrupted power upshifting and downshifting between gears is available along with the high mechanical efficiency of a manual transmission being available in an automatic transmission. Significant increases in fuel economy and vehicle performance are achieved. The present invention is directed to an electro-mechanical automatic transmission for use with a front wheel drive vehicle. A similar design for a rear wheel drive vehicle is disclosed in commonly assigned U.S. Pat. No. 5,966,989 which is herein incorporated by reference.
According to the present invention, an electro-mechanical automatic transmission is provided including a first input shaft and a second input shaft concentric with the first input shaft. A plurality of drive gears are rotatably mounted to the first input shaft. An idler gear is provided in meshing engagement with an input gear on said second input shaft. The idler gear drives a lay shaft which is also provided with a plurality of drive gears rotatably mounted thereon. The first input shaft and the lay shaft are provided with synchronizer devices for selectively engaging the drive gears thereto. A driven shaft is provided with a plurality of driven gears in meshing engagement with the drive gears. The electro-mechanical automatic transmission is provided with a pair of electro-mechanical clutch actuators for selectively disengaging dual clutches which transmit engine torque to the input shafts, as well as an electro-mechanical shift actuator system which operatively engage the synchronizer devices for selectively engaging the drive gears.
The dual clutch system of the present invention includes two dry discs driven by a common flywheel assembly. Two electro-mechanical clutch actuators are provided to control disengagement of the two-clutch discs independently. Shifts are accomplished by engaging the desired gear prior to a shift event and subsequently engaging the corresponding clutch.
The transmission of the present invention can be in two different gear ratios at once, but only one clutch will be engaged and transmitting power. To shift to the new gear ratio, the driving clutch will be released and the released clutch will be engaged. The two-clutch actuators perform a quick and smooth shift as directed by an on-board vehicle control system using closed-loop control reading engine RPMs or torque. The transmission shaft that is disengaged will then be shifted into the next gear ratio in anticipation of the next shift.
A hill holder mechanism is provided in the form of an engagable overruning one-way roller clutch. This clutch will be engaged when the transmission is in first, second, or third gears in order to prevent vehicle rollback on a hill. A series of four synchronizer pairs are preferably used on one of the input shafts and a lay shaft. The hill holder mechanism is selectively engaged by one of the synchronizers. The hill holder prevents rollback of the vehicle when it is stopped. Contrary to an automatic transmission, there is no engine torque required to hold the vehicle from rolling back on a grade, thus efficiency is improved.
The transmission of the present invention includes drive gears on a first and a second shaft in meshing engagement with common driven gears on the output shaft. The ability to utilize a single driven gear to mate with two different drive gears on the first and second shafts allow for the reduction of the number of gears and a reduction in size of the transmission. The shortened length of the transmission permits the transmission to be utilized in a transaxle powertrain.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.